1. Field of the Invention
The disclosures herein generally relate to base stations for mobile communication systems, and particularly relate to a base station for a mobile communication system in which a mobile station selects a code from a plurality of codes for transmission, and the base station allocates bandwidths for handover ranging or bandwidth request ranging in response to the transmitted code.
2. Description of the Related Art
In recent years, IEEE (Institute of Electrical and Electronic Engineers) has been engaged in an effort to standardize a mobile communication system referred to as WiMAX (Worldwide Interoperability for Microwave Access).
WiMAX includes IEEE802.16d that is a standard for a stationary subscriber station and IEEE802.16e that is a standard for a mobile station.
The disclosures herein relate to a ranging method for use in IEEE802.16e, i.e., the standard for mobile stations, for example. The term “ranging” refers to the adjustment of transmission timing and PHY (i.e., physical layer) functions (such as adjustment of a transmission power level) performed by a mobile station based on checking radio communication conditions between the mobile station and a base station.
IEEE802.16e defines several PHY systems. The following purpose-specific ranging procedures are provided for an OFDMA (Orthogonal Frequency Division Multiple Access) PHY:    (1) Initial Ranging: ranging performed by a mobile station when it first connects to a base station;    (2) Periodic Ranging: ranging performed at constant intervals by a mobile station that is already connected to a base station;    (3) Bandwidth Request Ranging (BR Ranging): ranging performed by a mobile station when it requests a bandwidth; and    (4) Handover Ranging: ranging performed by a mobile station performing handover with respect to a base station that is to be a handover destination.
FIG. 1 is a drawing showing an example of a frame configuration for an OFDMA physical layer. In FIG. 1, the horizontal axis represents the OFDMA symbol number, which corresponds to a time axis. The vertical axis represents the subchannel logical number.
The OFDMA frame includes a DL (i.e., downlink: downward link from a base station to a mobile station) subframe, a UL (i.e., uplink: upward link from a mobile station to a base station) subframe, a TTG (Transmit/receive Transition Gap), and a RTG (Receive/transmit Transition Gap).
The DL subframe includes a preamble, an FCH (Frame Control Header), a DL-MAP, a UL-MAP, and a plurality of DL bursts.
The preamble includes a preamble pattern required by a mobile station to establish frame synchronization. The FCH includes information about subchannels used and information about the DL-MAP that is provided at the immediately following position. The DL-MAP includes mapping information regarding the DL bursts in the DL subframe. By referring to this information, a mobile station can identify a UL-MAP (transmitted on a DL burst #1) and DL bursts #2 through #6 shown in FIG. 1.
The UL-MAP includes mapping information regarding the ranging regions and UL bursts in the UL subframe. By referring to this information, a mobile station can identify the ranging regions and UL bursts #1 through #4 shown in FIG. 1.
The burst is an area throughout which the same modulation scheme and the same FEC (Forward Error Correction) are used in combination. The DL-MAP/UL-MAP specifies a combination of a modulation scheme and an FEC for a given burst. The burst carries various control messages defined in IEEE802.16e and user data.
For performing ranging, a mobile station first randomly selects a code from 256 PN (i.e., pseudo noise) codes each having a 144-bit length, and modulates the code by BPSK (Binary Phase Shift Keying) for transmission to a base station by use of a ranging region shown in FIG. 1. This code is referred to as a “ranging code” or “CDMA ranging code” (CDMA: Code Division Multiple Access).
In FIG. 1, a ranging region for initial ranging or handover ranging and a ranging region for periodic ranging or bandwidth request ranging are separately allocated. A mobile station uses either region for transmission of a ranging code in accordance with the purpose of ranging. Each ranging region is allocated as shown in FIG. 1 as defined by the UL-MAP.
In the following, procedures for bandwidth request ranging and handover ranging will be described.
FIG. 2 is a drawing showing the configuration of a typical network. The configuration shown in FIG. 2 includes mobile stations 1 through 4, base stations 11 and 12, a control apparatus 21, and a core network 22. The control apparatus 21 manages and controls a plurality of base stations, and serves to connect between the base stations as well as between the base stations and the core network.
In FIG. 2, the mobile station 1 is performing handover from the base station 11 functioning as a serving base station (serving BS) to the base station 12 that is a target base station (target BS). The mobile station 2 attempts to handover from the base station 11, but cancels the handover to return to the base station 11. Procedures for handover ranging and bandwidth request ranging in these cases will be described by referring to FIG. 3 and FIG. 4.
FIG. 3 is a drawing showing the procedure for related-art handover ranging.
In step S1, the mobile station 1 transmits a handover request (i.e., MOB_MSHO-REQ message) including an indication of a plurality of candidate target base stations to the base station 11.
In step S2, the base station 11, the control apparatus 21, and the base station 12 exchange required backbone messages upon receiving the handover request.
In step S3, the base station 11 transmits a handover response (i.e., MOB_BSHO-RSP message) including an indication of a plurality of candidate target base stations capable of handover to the mobile station 1. The target base stations identified in step S3 may possibly be different from the target base stations identified in step S1 as a result of the exchanges of backbone messages.
In step S4, the mobile station 1 transmits, to the base station 11, a handover indication (i.e., MOB_HO-IND) indicating that the base station 12 is selected as the target base station. In so doing, HO_IND_type of MOB_HO-IND is set to “Serving BS release”, so that the connection between the mobile station 1 and the base station 11 will be disconnected.
In step S5, the base station 11, the control apparatus 21, and the base station 12 exchange required backbone messages upon receiving the handover indication.
In step S6, the mobile station 1 transmits a handover ranging code during a ranging slot of a ranging region shown in FIG. 1 to the base station 12. The 256 ranging codes previously described have their usages defined on a purpose-specific basis (i.e., for initial ranging, periodic ranging, bandwidth request ranging, and handover ranging). In step S6, a ranging code defined for handover is used. The purpose-specific usage of a ranging code is defined by a UCD (Uplink Channel Descriptor) message that is periodically broadcast from a base station. The base station 12 identifies the purpose of ranging based on the received ranging code.
FIG. 5 is a drawing showing an example of the configuration of a ranging region. What is shown in FIG. 5 corresponds to the detail of a ranging region shown in FIG. 1. In FIG. 5, N1 represents OFDMA symbol numbers for a single ranging slot, and N2 represents subchannel numbers for a single ranging slot. A remainder of the region that does not have a sufficient number of symbols to constitute a single ranging slot is not allocated to a ranging slot, and is not used (shown as “Empty” in FIG. 5). In FIG. 5, a ranging slot number is shown in each ranging slot, which contains a ranging code.
A mobile station transmits a ranging code by use of one of the ranging slots. As a general principle, the mobile station randomly selects a ranging code and a ranging slot.
In step S7, the base station 12 having received the ranging code broadcasts a ranging response message (RNG-RSP) to request the adjustment of timing and transmission power level (time/power corrections) to the mobile station 1. The mobile station 1 can determine whether it is the intended recipient of the broadcast RNG-RSP by checking an “original ranging frame/code/slot” contained in the RNG-RSP. The “ranging frame” indicates 8 LSBs (least significant bits) of the frame number of the frame that is used by the mobile station to transmit the ranging code. The “ranging code” indicates the index value of the ranging code transmitted by the mobile station. The “ranging slot” indicates the subchannel logical number and symbol number of the ranging slot that is used by the mobile station to transmit the ranging code. By checking these parameters, the mobile station determines whether it is the intended recipient of the broadcast RNG-RSP.
In step S8, the base station 12 having received the ranging code broadcasts a CDMA_Allocation_IE that is one of the information elements in the UL-MAP, thereby allocating a bandwidth of the UL to the mobile station 1. The UL-MAP is comprised of various types of information elements. The mobile station 1 can determine whether it is the intended recipient of the broadcast CDMA_Allocation_IE by checking an “original ranging frame/code/slot” contained in the CDMA_Allocation_IE.
In step S9, the mobile station 1 transmits a ranging request message (RNG-REQ) inclusive of its own MAC Address to the base station 12 by utilizing the bandwidth allocated in step S8 to the UL burst. This RNG-REQ is transmitted by using the UL burst rather than using a ranging region.
In step S10, the base station 12 transmits a RNG-RSP inclusive of the MAC Address of the mobile station 1, thereby allocating a Basic/Primary CID (Connection ID) to the mobile station 1.
In step S11 and subsequent steps, network entry processing for the mobile station 1 will be performed by exchanging various control messages by use of the above-noted Basic/Primary CID.
FIG. 3 described above shows the procedure for handover initiated by a mobile station (i.e., MS Initiated HO). As another procedure, there is a procedure for handover initiated by a network (i.e., Network Initiated HO). In the case of network-initiated handover, MOB_MSHO-REQ in step S1 is nonexistent, and a base station transmits an MOB_BSHO-REQ (i.e., handover request) to a mobile station in place of the MOB_BSHO-RSP transmitted in step S3. Other than these changes, the procedure for handover is the same as the procedure for mobile-station-initiated handover. It should particularly be noted that the ranging process is exactly the same.
FIG. 4 is a drawing showing the procedure for related-art bandwidth request ranging in the case of handover canceling. A description of the points that are the same as in FIG. 3 will be omitted. Steps S1 through S5 of FIG. 4 are the same as those shown in FIG. 3.
In step S6, the mobile station 2 transmits a bandwidth request ranging code during a ranging slot of a ranging region shown in FIG. 1 to the base station 11. The mobile station 2 is about to return to the base station 11 upon canceling handover to the base station 12. The base station 11 has started Resource Retain Timer upon receiving the handover indication (MOB_HO-IND), and retains the contexts regarding the mobile station 2 until the timer expires. Before the timer expires, thus, the mobile station 2 can return to the base station 11 by transmitting a handover cancel message as will later be described in connection with step S10. In this case, an initial connection procedure that is typically required does not have to be performed.
In step S7, the base station 11 transmits CDMA_Allocation_IE, thereby allocating a bandwidth of a UL burst to the mobile station 2.
In step S8, the mobile station 2 utilizes the allocated bandwidth of a UL burst to transmit a bandwidth request required to transmit a handover cancel message. There is no need to transmit this bandwidth request if a bandwidth sufficient for transmitting a handover cancel message is already allocated in step S7. The base station 11 needs to allocate, through the CDMA_Allocation_IE, a bandwidth sufficient to transmit a bandwidth request, but there is no guarantee that a wider bandwidth is allocated.
In step S9, the base station 11 allocates a bandwidth of a UL burst to the mobile station 2 through UL-MAP_IE.
In step S10, the mobile station 2 utilizes the allocated bandwidth to transmit a handover cancel message (MOB_HO-IND (HO_IND_type=HO cancel)).
In step S11, the base station 11, the base station 12, and the control apparatus 21 exchange backbone messages to perform cancel processing in the network. With this, the returning of the mobile station 2 to the base station 11 is completed.
Japanese Patent Application Publication No. 2007-504747 discloses allocating a ranging code on a ranging-purpose-specific basis as previously described and allocating a UL burst bandwidth preferentially for HO ranging, for example, thereby increasing the speed of handover process.
Japanese Patent Application Publication No. 2003-318859 discloses determining the number of ranging codes and back-off value assigned on a ranging-purposes-specific basis in response to congestion level in the network.
Japanese Patent Application Publication No. 2006-5946 discloses a target base station that transmits a Fast_Ranging_IE to a mobile station to allocate a bandwidth for the mobile station to transmit a RNG-REQ message.
A ranging code and a ranging slot are randomly selected in the related-art CDMA-based ranging scheme. Contention thus occurs if another mobile station selects the same ranging code and the same ranging slot.
In the case of the occurrence of contention, a mobile station detects the occurrence of contention based on whether a retransmission timer expires, and, then, waits for the passage of a back-off time by applying the back-off algorithm defined in IEEE802.16e, followed by retransmitting the ranging code. Such procedure results in a delay in handover process. FIG. 6 is a drawing showing a related-art procedure for handover ranging in the case of occurrence of contention. For the sake of convenience, the same step numbers are used in FIG. 3 and FIG. 6.
By the same token, the procedure for bandwidth request ranging suffers a problem in that the handover cancel process is delayed upon the occurrence of contention between the mobile station 2 and the mobile station 3 shown in FIG. 2, for example.
The method disclosed in Japanese Patent Application Publication No. 2007-504747 can increase the speed of handover process if no contention occurs, but cannot prevent the occurrence of contention.
The method disclosed in Japanese Patent Application Publication No. 2003-318859 can reduce the number of the occurrences of contention, but cannot prevent the occurrence of contention.
The method disclosed in Japanese Patent Application Publication No. 2006-5946 transmits a RNG-REQ message of step S9 without performing CDMA-based ranging shown in FIG. 3, thereby increasing the speed of handover process. In order to use this method, however, the mobile station needs to perform a process called “association” with the target mobile station while the mobile station is still connected to the serving base station to receive an instruction for time/power corrections from the target mobile station. There is also a need that this instruction be not yet expired while the mobile station is still connected to the base station. Further, the “association” function is optional in the Mobile WiMAX System Profile. It is thus highly probable that a mobile station and a base station are not provided with this function.
Accordingly, there is a need for a base station of a mobile communication system that can prevent the occurrence of contention associated with handover ranging and bandwidth request ranging.